1. Field of the Invention
The present invention relates generally to automatic gain controls (AGCs) and, in particular, to a method of offset voltage reduction for AGCs.
2. Background Art
Electronic circuits are often used to process signals. It is often desirable to provide signals at a predictable, fixed amplitude or level. However, such signals are not always available. Thus, it is useful to be able to adjust the amplitude of signals to maintain a desired level. An AGC circuit may be used to perform this adjustment. An AGC circuit includes an amplifier capable of providing variable gain. The AGC circuit monitors the output of the variable gain amplifier and compares the amplitude of the output to a reference level. If the amplitude of the output exceeds the reference level, the AGC circuit reduces the gain of the variable gain amplifier. If the amplitude of the output is less than the reference level, the AGC circuit increases the gain of the variable gain amplifier. By adjusting the gain of the variable gain amplifier, the AGC circuit provides an output having a predictable amplitude range.
AGC circuits are typically used to amplify AC signals, pulsed DC signals or other time dependent signals. When the input signal is removed, it is desirable for the output to be zero. However, device mismatches and other nonideal conditions may result in an AC signal being superimposed on a DC offset. The DC offset is typically affected by changes in the temperature and/or gain of the AGC. Even if the AC signal is removed or set to a zero level, the DC offset is still present at the output as a nonzero level. The DC offset is not representative of the AC signal applied to the amplifier and typically increases the total harmonic distortion of the AGC circuit. To ensure accurate, undistorted amplification of the applied input signal, the DC offset should be removed. To eliminate this DC offset, a method of offset voltage trim is needed.
In the past, typically, the output of an AGC circuit was differentially AC coupled to other circuitry to remove DC offsets. The AC coupling required the addition of four pins to an integrated circuit (IC) package, thereby increasing the size and complexity of the IC. Two external capacitors were also required to provide AC coupling of a differential AGC output signal. The two capacitors were coupled to the additional four pins of the IC, AC coupling the differential AGC output pins to the respective differential input pins of the subsequent circuitry. The increased number of components and complexity of connections increased the difficulty and cost of manufacturing such an AGC circuit and decreased its reliability. Thus, a more efficient and reliable method of reducing the DC offset of an AGC circuit is needed.